1. Field of the Invention
The present invention relates to a backlight module and particularly to a backlight module which has two optical sheets.
2. Description of the Prior Art
In recent years, the traditional cathode ray tube display (commonly called CRT display) has been gradually replaced by a liquid crystal display (LCD). The main reason is that the LCD releases far less radiation than the CRT display. Moreover, the production cost of the LCD has dropped significantly in recent years. In general, the LCD includes a backlight module and a liquid crystal panel. The backlight module mainly aims to provide a light source for the LCD.
Refer to FIG. 1 for a conventional backlight module 100 which includes a case 110, a cold cathode fluorescent lamp (CCFL) 120, a diffusion plate 130, an optical film 140 and a plurality of needles 150. The CCFL 120 is located in the case 110 which has a reflective sheet (not shown in the drawing) on the inside surface to use light generated by the CCFL 120 more effectively. The diffusion plate 130 mainly diffuses the light generated by the CCFL 120 so that the light can be projected more evenly to the liquid crystal panel (not shown in the drawing) without creating uneven luminosity on the display of the LCD. The needles 150 aim to support the diffusion plate 130 to prevent it from warping due to its own weight.
In addition, the optical film 140 further includes a brightness enhancement film (BEF) 144. As the diffusion plate 130 cannot fully overcome the uneven luminosity, a diffusion film 142 is added to evenly diffuse the light. Moreover, light exited from the diffusion film 142 has a greater light exit angle. Hence, the BEF 144 is disposed on the diffusion film 142. The BEF 144 has a thickness about 0.062 mm to 0.375 mm and a plurality of prism structures 144a formed at an upper side thereof to converge light. Thus, the exit angle of the light emitting from the BEF 144 becomes smaller. Therefore, the luminosity within the visual angle of the backlight module 100 is enhanced.
Due to manufacturing process and material, the BEF 144 often is the most expensive item in the backlight module 100. In order to reduce the total cost of the backlight module 100, some producers use a less expensive prism plate to replace the BEF 144. However, the prism plate is thicker and, thus, the total thickness of the backlight module 100 also becomes thicker. Hence, how to use the prism plate without increasing the thickness of the backlight module 100 is an issue remained to be resolved in the industry.